Method for forming an isolating plug

ABSTRACT

Method of the subject disclosure can be used in the oil and gas production industry, in particular, for plugging fractures in the near-wellbore zone during the fracturing fluid removal, as well as for plugging different kinds of fractures and branches in the casing. For formation of an isolating-plug, a catch is placed upstream of the point where the isolating plug shall be formed, so as to prevent the passing of fibers and to promote the forming of fiber aggregates. A fluid containing dispersed fibers is then injected into a well, and the catch is periodically opened to release the fiber aggregates and to allow their free flow to the location where the plug shall be formed.

The invention relates to the oil and gas production industry, in particular, to methods for isolating near-wellbore zones and fractures, and can be used for plugging fractures in the near-wellbore zone during the fracturing fluid removal, as well as for plugging different kinds of fractures and branches in the casing.

Hydraulic fracturing is the main tool used for increasing the productive capacity of a well through creation or expansion of channels from a wellbore to a producing formation. This operation is generally accomplished by injecting a fracturing fluid into a wellbore which intersects an underground deposit, and by exposing the strata to the fluid pressure action. In order to increase the oil and gas production rates, it is necessary to solve the problem of how to remove the fracturing fluid and to plug the near-wellbore zones and fractures. There are a few methods for solving this problem, and these methods are usually based on addition of solid inclusions to fracturing-fluid solutions. The formation of an isolating plug starts from the formation of a bridge (so-called “bridging”) which is nothing but a cluster of solid inclusions stably captured from the solution on the fracture surface. At the same time, the fluid keeps on flowing through the fixed agglomerate of solid inclusions. As a result, the solids-containing solution (the slurry) is filtered, which gradually increases the density of the solids arrested and reduces the penetrability of the resulting structure and completely stops the flow. For example, U.S. Pat. No. 7,036,588, describes the use of ceramic particles and starch buildups for fluid loss control purposes; U.S. Pat. No. 7,318,481 describes shape-memory foams which are used as a withdrawal agent; Application No. WO2007066254 describes the reversible plugging of a fracture or a well with a degradable material. U.S. Pat. No. 7,331,391 describes the use of water-soluble fibers for drilling-mud loss control purposes.

Patent No. RU 2330931 describes a device which acts as a temporary plug and which consists of a fiber layer, a fiber-collection element, a grid or a perforated material (made of fabric) and a downhole body reamer (a spring-type or umbrella-type mechanism). By using this device, it is possible to simplify the placement of a packer in a well. The packer placed in the wellbore accumulates fibers from the overlying fiber-containing area, thus forming an impermeable plug in the wellbore. This method has a number of limitations on use, namely: relative design complexity and the fact that the plug is formed in the wellbore, which makes the access to the well areas behind the packer (access to the well end) difficult or impossible.

A high fiber concentration is required for successful formation of a plug from fibers. Such an approach encounters a number of problems, namely: financial expenses related to the production/purchase/transportation of large amounts of fibers, and expenses related to the expansion of injection equipment capacities. At the same time, processing a high fiber concentration may break down the equipment (pumps, mixers, etc.).

The technical result achieved with the implementation of the invention consists in providing efficient isolation of fractures in the near-wellbore zone, while reducing the fiber concentration and preventing the pumps and other equipment from clogging.

For achievement of the said technical result, we suggest a method for forming an isolating plug, the method comprising the following steps: a catch is placed upstream of a location of the insulating plug being formed so as to prevent passing of fibers and to promote fiber aggregates forming; injecting a fluid containing dispersed fibers into a well; and the catch is periodically opened to release the formed fiber aggregates and to allow their free flow to the location of the insulating plug being formed.

The catch which prevents the fiber passing can be made in the form of a grid which crosses the fluid flow and which can be removed during the fluid injection process.

The catch which prevents the fiber passing can be made in the form of a rotatable plate which is placed across the fluid flow, some circular sectors of which have fluid-permeable (but fiber-impermeable) holes and other sectors are opened.

The catch which prevents the fiber passing can also be made in the form of an element which is placed across the fluid flow and allows the fluid passing but prevents passing of fibers and promotes fiber aggregates forming.

The invention is illustrated with drawings, where

FIG. 1 shows the scheme for implementation of the method for forming an isolating plug, according to this invention, and

FIG. 2 shows optional embodiments of the trap preventing the fiber passing.

The method for forming an isolating plug is implemented as follows. Let us consider the flow of a fiber-containing solution (a fiber slurry), e.g. a fracturing fluid (FIG. 1). The flow direction is shown with an arrow (3). At the initial moment, fibers (1) are uniformly distributed in the solution. For fracture bridging purposes, the fiber concentration shall be high enough and shall be determined from the flow conditions and the fracture size (usually, a fiber concentration of 100 ppt (pounds per thousand tons) is recommended for the fracture plugging in clay rock). When the fluid-permeable (but fiber-impermeable) catch (2) is closed, the fiber filtration process starts in the catch, with pure (fiberless) fluid flowing further. This will locally increase the concentration of the fibers (1). The fibers around the catch (2) will form a more dense three-dimensional grid.

Then, if you open the catch (2) and allow the formed grid to flow, it will move on as a separate aggregate (4) the local concentration of which will be much higher than the initial concentration of the slurry. The size and the density of the generated aggregates (4) will depend on the time during which the catch was closed, and on the slurry flow velocity. That is, the time during which the catch is closed shall be selected, based on the conditions of the problem posed (i.e. the flow velocities, the initial fiber concentration, the fiber size, and the size of fractures to be plugged). When periodically repeated, such processes produce a series of fiber aggregates which move along with the fluid and which allow an increase in the probability of bridging.

The optimum shape of the catch is a simple holed plate or a grid permeable to fluid but impermeable to fibers. FIG. 2 shows a few potential embodiments of a similar device. Option A is represented by a swing-type grid (2) on the end of the pipe (1), which can be periodically opened and closed to form fiber aggregates during the injection process. Option B is represented by a rotating plate (3) some sections of which (e.g. two sections, as shown in the figure) are opened, while others are holed plains. As the catch rotates, the produced aggregates are discharged into the slurry. Option C is represented by a catch in the form of an element (4) which is periodically inserted into and pulled out of the pipe. Fibers start accumulating on the element (4), which results in the formation of aggregates. After an aggregate of the required size has been formed, the element is pulled out of the pipe, thus allowing the aggregate to move on.

Option C was selected as a pilot experiment to check the method under laboratory conditions. The element to be placed across the fluid flow to prevent the fluid from flowing was made in the form of a fork and was inserted/pulled out through the holes in the pipe where the fiber slurry was flowing. Slurries at different concentrations were pumped at different rates through the aggregate generator. The slurry was prepared from an aqueous guar gel and polylactic acid fibers. The guar concentration varied from 10 to 60 ppt (pounds per thousand tons), while the fiber concentration varied from 30 to 120 ppt (pounds per thousand tons). The injection rate varied from 50 ml/min to 250 ml/min. A total of 3.5 litres of slurry were pumped. As a result of the aggregate generation, the probability of plug formation increased drastically.

So, the suggested method for forming an isolating plug allows a local increase of the fiber concentration of the slurry by generating mechanically fiber aggregates (“flocks”). Periodical temporary stops of the fibers dispersed in the fluid, accompanied by the filtration of the carrying fluid through the fiber grid, will allow the production of aggregates of a specified size and with a specified fiber content. Depending on the initial fiber concentration and the slurry flow velocity, mechanical generation of flocks will play a crucial role in the plug formation process. In situations where the fiber concentration is low and/or the slurry flow velocity is high, separate fibers or small aggregates will not be able to form a plug in a fracture, so the artificial generation of aggregates of a specified size and with a specified fiber content becomes very important.

Using the presented technology will allow the production of a plug at a low initial fiber concentration (the surface concentration, when considering the formation of a plug in a fracture) due to the presence of mechanically generated aggregates. Similar generation can be carried out at any stage (in any point) of the slurry flow. Using degradable or non-degradable fibers will allow you to produce either a permanent or a temporary plug. The catch (the aggregate generator) can be integrated into the casing in any point of the well from the surface (the well head) to the point in immediate proximity to the area where the plug shall be formed. The catch can also be installed on the surface in underwater pipes downstream of pumps and mixers. 

1. A method for forming an insulating plug, comprising the following steps: placing a catch upstream of a location of the insulating plug being formed, the catch preventing passing of fibers and promoting fiber aggregates forming; injecting a fluid containing dispersed fibers into a well, and periodically opening the catch to release the formed fiber aggregates and to allow their free flow to the location of the insulating plug being formed.
 2. The method of claim 1 wherein the catch is made as a grid which can be periodically removed during the fluid injection process.
 3. The method of claim 1 wherein the catch is made as a rotatable plate disposed across the fluid flow, some sections of which are provided with holes, allowing the fluid passing but preventing passing of fibers, the others sections are open.
 4. The method of claim 1 wherein the catch is made as an element disposed across the fluid flow allowing the fluid passing but preventing passing of fibers and promoting fiber aggregates forming.
 5. The method of claim 1 wherein the fluid is a fracturing fluid. 